Ever since the potential for generating electrical power from nuclear reactions was recognized, scientists have strived to devise the best methods of harnessing nuclear power and putting it to practical use. The main objectives of such research have been to create the most efficient methods of power conversion, power converters that can generate electrical power from nuclear power sources for sustained periods of time without maintenance, and smaller, more manageable power converters that can be used as everyday power sources. The sources of nuclear energy that scientists have sought to harness include nuclear fission (the splitting of atoms), radiation (the emission by radiation of alpha, beta or gamma rays) and nuclear fusion (the fusing of atoms). The present invention is designed to generate electrical power from energy produced from nuclear fission and/or radiation. For the purposes of this document, the following terms shall have, in addition to their generally accepted meaning, the meanings listed below:
(a) the term “nuclear material” or “nuclear materials” refers to fissile materials and radioactive isotopes that are non-fissile, but produce radiation—either alpha, beta or gamma type radiation;
(b) the term “fissile material” includes uranium, plutonium, thorium, neptunium and mixtures of plutonium and uranium;
(c) Uranium refers to the following classifications—depleted uranium (U-235 concentration less than 0.7%), natural uranium (U-235 concentration equal to approximately 0.7%), low enriched uranium (U-235 or U-233 concentration less than 20%), high enriched uranium (U-235 or U-233 greater than 20%);
(d) Plutonium refers to reactor grade plutonium where the Pu-240 concentration is nominally 10% to 15%.
The best-known method of generating electrical power using nuclear energy is via heat exchange processes, the method used in nuclear power plants to generate electricity for use in the United States national grid. In the nuclear power plant, rods of uranium-235 are positioned in a reactor core where fission, the splitting of the uranium-235 atoms, occurs. When the uranium-235 atom splits apart, large amounts of energy are emitted. Inside the nuclear power plant, the rods of uranium are arranged in a periodic array and submerged in water inside a pressure vessel. The large amount of energy given off by the fission of the uranium-235 atoms heats the water and turns it to steam. The steam is used to drive a steam turbine, which spins a generator to produce electrical power. In some reactors, the superheated water from the reactor goes through a secondary, intermediate heat exchanger to convert water to steam in the secondary loop, which drives the turbine. Apart from the fact that the energy source is uranium-235, the nuclear power plant uses the same power conversion methods found in power plants that burn fossil fuels.
Nuclear power plants, in general, have energy conversion rates of between 30 and 40 percent. This efficiency rate is very good considering that several steps are used in such power plants to convert the nuclear energy to electrical energy. Consequently, nuclear power plants are a good source for large-scale generation of electricity. However, apparatus that use heat transfer techniques for generating electricity from nuclear energy are, in general, large and inefficient for small-scale power conversion.
Research has been performed into ways of reducing the size of the equipment necessary for an effective heat transfer system for generating electrical power from nuclear materials. Some success has been achieved, and since the 1950s small nuclear power plants have powered a great number of military submarines and surface ships. However, because of the associated risks, heat transfer systems have not been used for other small-scale energy sources and are no longer used on United States space vehicles. The use of nuclear energy to power nuclear submarines highlights the advantages that nuclear materials have as a power source; for example, a nuclear submarine can travel 400,000 miles before needing to be refueled.
Because of the potential of nuclear materials as a source for providing energy over a long period of time, a great deal of research has been performed to develop a small, self-contained power source utilizing nuclear materials that does not have the associated risks inherent in a heat transfer system. This research has led to the development of several methods of converting nuclear energy into electrical energy.
Theoretically, the best methods for converting nuclear energy into electrical energy should be direct methods where the nuclear energy is directly changed into electrical energy. The nuclear power plant discussed above involves an indirect, two-step process in which the nuclear energy is transferred into thermal energy that causes water to turn to steam that is used to drive turbines and create electrical energy. Direct conversion methods are potentially the most efficient conversion methods because they would avoid the inherent energy loss during each conversion process. The following are examples of direct conversion techniques that have been proposed up until the present date.
Conversion of nuclear energy to electrical energy using solid semiconductors. In this process, radiation energy from the radioactive isotope is directly converted to electrical energy by irradiating a semiconductor material with radioactive decay products to produce a number of electron-hole pairs in the material. To accomplish this, nuclear material, such as a radioactive isotope, is placed in close proximity to a solid semiconductor. As it decays, the radioactive isotope produces radiation. Because it is in close proximity with the solid semiconductor, some of this radiation enters the solid semiconductor and causes electron-hole pairs to be generated. Generally, the solid semiconductor is configured so as to incorporate a p-n junction that contains a built-in electric field within a region called the depletion region. This electrical field applies a force that drives electrons and holes generated in the depletion region in opposite directions. This causes electrons to drift toward the p type neutral region and holes toward the n type neutral region. As a result, when radiation enters the solid semiconductor, an electrical current is produced. Current can also be generated from electron-hole pairs produced within a few diffusion lengths of the depletion region by a mechanism involving both diffusion and drift. A Schottky barrier junction formed on either an n-type or p-type semiconductor can also be used in lieu of the p-n junction. In that case, an analogous process occurs when the metal on the n-type (p-type) semiconductor collects drifting holes, as did the p-type (n-type) neutral region in the p-n junction.
The potential conversion efficiency of the solid semiconductor system is high. However, the solid semiconductor method of converting nuclear power cannot be used to produce large power outputs for extended periods of time because the high energy radiation that enters the solid semiconductor also causes damage to the semiconductor lattice. Furthermore, if the energy source is fissile material, some of the fragments of fissile material that enter the solid semiconductor remain in the solid semiconductor. The introduction of trace amounts of defects, including native and impurity point defects and extended defects, can significantly reduce semiconductor device performance. Over time the solid semiconductor is degraded and efficiency decreases until it is no longer useful for power conversion. Consequently, even though systems using solid semiconductors as direct converters of nuclear energy to electrical energy are potentially very efficient, they are often impractical for high power, long duration applications.
Conversion of nuclear energy to electrical energy using Compton scattering. Compton scattering occurs when high-energy gamma radiation interacts with matter causing electrons to be ejected from the matter. A method for direct conversion of nuclear energy to electrical energy has been proposed in which a gamma radiation source is surrounded by an insulating material. As a result of Compton scattering, the gamma rays interact with the insulating material and cause electrons to be produced. These electrons can be collected to produce an electric current. Experiments to date have not been able to demonstrate that this method can generate sufficiently large amounts of electricity with the required efficiency and reliability at a sufficiently low cost to be useful for widespread use in practical applications.
Conversion of nuclear energy to electrical energy using an induction process. The use of induction to convert nuclear energy to electrical energy involves apparatus that provides electrical power by modulating the density of a cloud of charged particles confined within an enclosed space by a magnetic field. A radioactive material is positioned at the center of an enclosing hollow sphere having its inner surface coated with a metal, such as silver. The sphere is centrally positioned between the poles of a permanent magnet. As the radioactive material decays it emits radiation that in turn causes the cloud of charged particles to move. The movement of the charged particles results in a variation in the density of the cloud of charged particles and a variation in the magnetic field created by the cloud. This variation in the magnetic field induces an electric current in a conductive wire. Once again, the conversion efficiency of the system is very low and the amount of electrical power provided is too small for most applications.
Conversion of nuclear energy to electrical energy using thermoelectric systems. Thermoelectric conversion systems rely on direct conversion of thermal energy to electricity by means of the Seebeck effect. The Seebeck effect describes the phenomenon that when a thermal gradient occurs in a system containing two adjacent dissimilar materials, a voltage can be generated. Therefore, if radioactive material is placed in proximity to the system, the radiation produced by the radioactive material will heat the material causing a thermal gradient and as a result of the Seebeck effect, a voltage difference can be generated. A load can be inserted into the system, allowing electrical power to be removed from the system. Thermoelectric converters are used in radioisotope thermoelectric generators for deep space probes and can provide up to a kilowatt of power. However, theoretical conversion efficiencies for commonly used materials are only 15-20 percent and in practice, conversion efficiencies are much lower.
Conversion of nuclear energy to electrical energy using thermionic systems. Thermionic systems make use of the physical principle that certain materials when heated will emit electrons. Thermionic systems use nuclear matter, radioisotopes or fissile material, as an energy source to heat an emitter cathode that emits electrons which can be collected on an anode surface, delivering electrical power to an external load. Conversion efficiencies for thermionic systems increase with emitter temperature, with theoretical efficiencies ranging from 5% at 900 K to over 18% at 1,750 K. The drawbacks of the thermionic conversion system are poor efficiencies, high operating temperatures, and intense radiation environments.
Conversion of nuclear energy to electrical energy using fluorescent materials. In this system, a mixture of a radioactive substance and a fluorescent material is positioned between a pair of photovoltaic cells. The radioactive substance produces radioactive rays that excite the atoms of the fluorescent material and cause it to emit photons. The photovoltaic cells use this radiation to generate electricity. In general, this system requires a very complex structure but nevertheless provides poor conversion efficiency on the order of less than 0.01%.